1. Field of the Invention
The invention relates to friction clutches for establishing and disestablishing torque flow paths in a hybrid electric vehicle powertrain.
2. Background Art
A hybrid electric vehicle powertrain with an internal combustion engine and a high voltage electric motor arranged in series is disclosed in U.S. Pat. No. 6,585,066. A powertrain of that type sometimes is referred to as a modular hybrid transmission when an electric motor portion of the powertrain and a discrete ratio transmission mechanism comprise separate modules that are integrated in a compact assembly capable of delivering driving torque through both an electromechanical torque flow path and a fully mechanical torque flow path. The torque capacity of one path supplements the torque capacity of the other. A fully electric driving torque flow path may be obtained using the motor solely.
In the powertrain disclosed in U.S. Pat. No. 6,585,066, the motor is in a pre-transmission arrangement in the assembly and the engine is connected through a damper to a disconnect clutch. The disconnect clutch is used to establish and disestablish torque flow from the engine to the input shaft of the discrete ratio transmission. A launch clutch connects the engine and the rotor of the electric motor to the input shaft of the transmission during a vehicle launch. The motor and the transmission are arranged to achieve engine stopping and starting modes, brake energy regeneration, an electric power boost for the engine and a fully electric drive.
In those instances when the engine is turned off, the motor can be used to drive the vehicle if the engine disconnect clutch is disengaged. When both clutches are engaged, the motor and the engine cooperate to deliver driving torque to the vehicle traction wheels through the transmission. When the launch clutch is used in this fashion, the clutch is allowed to slip when the vehicle speed is near zero. This will permit the engine to remain running if the engine is used as a power source, and it will permit the motor to continue turning when the motor is used as a power source.
During a launch, which is described in U.S. Pat. No. 6,974,402, dated Dec. 13, 2005, the launch clutch is controlled to effect a desired amount of slipping, which creates a substantial amount of thermal energy that must be dissipated. The dissipation of the thermal energy during launch requires the use of transmission cooling fluid. The cooling fluid must be supplied at very high flow rates during a launch event. Once the launch is complete, the launch clutch can be closed so that slippage is avoided. The clutch then does not create thermal energy that must be dissipated. Continued circulation of cooling fluid through the clutch assembly at this time is a waste of energy, which reduces the overall efficiency of the transmission.
Without knowledge of the actual oil temperature within the motor or the clutch, this clutch cooling fluid control heretofore has been done in an open loop fashion. To ensure robustness, the open loop control must be designed for a worst-case condition, which can cause it to be relatively inefficient when the operating conditions for the powertrain are normal and a high cooling flow rate is not needed. There is a need, therefore, for providing a more efficient control of cooling fluid flow.